1. Field of the Invention
The invention relates to the field of operational amplifier integrated circuits, and more particularly to a method and a circuit to operate a CMOS operational amplifier from a single 1.5V cell by biasing the transistors of the operational amplifier in the sub-threshold or weak inversion region of operation, in addition the class AB control circuit is novel and can be applied to any conventional low voltage Class AB output stage.
2. Description of the Related Art
Operational amplifiers (op amps) operating from a single cell must be designed with low voltage circuitry consuming very little power. It is difficult to have 1.5V operation in conventional CMOS processes using enhancement mode devices. This is because the minimum supply voltage required for rail to rail input common mode range is V.sub.GSN +V.sub.GSP +2V.sub.Dsat =0.7+0.8+2.times.0.2=1.9V. V.sub.GSN and V.sub.GSP are the gate source voltages for a NMOS and PMOS, respectively. These are typically 200 mV higher than V.sub.TN (=0.5V) and V.sub.TP (=0.6V) which are the threshold voltages of the NMOS and PMOS devices, respectively. V.sub.Dsat is the drain current saturation voltage of either NMOS or PMOS. The values specified are for a typical 0.25 um CMOS process.
Several solutions to the above problems associated with providing the weak inversion mode or low quiescent current have been proposed in the related art.
U.S. Pat. No. 5,568,093 (Holzer) discloses a Class AB amplifier using transistors operating in weak inversion mode. However, the application (amplifying a crystal oscillator output to CMOS logic levels) and, therefore, the disclosed circuit differs from the invention.
U.S. Pat. No. 5,057,789 (Nagaraj) describes a Class AB CMOS amplifier with low quiescent current but higher supply voltages are used.
U.S. Pat. No. 4,555,623 (Bridgewater et al.) shows a pre-amplifier for a focal plane detector array in which the devices are operated in a weak inversion region, but the circuit disclosed differs markedly from the invention.
It should be noted that none of the above-cited examples of the related art provide the combination of low voltage, low current consumption, high output current Class AB operational amplifier operating in weak inversion mode.
The invention overcomes the above problem by biasing the transistors in the so called "sub-threshold" or "weak inversion" region of operation. This lowers V.sub.GSN and V.sub.GSP to typically 200 mV below V.sub.TN and V.sub.TP and V.sub.Dsat to less than 100 mV. Therefore, the Op. Amp. can operate down to 0.3+0.4+2.times.0.1=0.9V. In addition, the sub-threshold operation results in a very low operating current. The invention describes an operational amplifier operating with a single 1.5V cell with a class AB output stage. Consuming only about 4 microAmpere (.mu.A) in the quiescent state, it can source and sink more than 60 uA into a 10K load resistance handling rail to rail input common mode and output signal swings. The class AB control circuit is novel and can be applied to any conventional (normal biasing--other than weak inversion) low voltage Class AB output stages.